Fluorescence Emissions Detector

ABSTRACT

A light source is gated ON and OFF in response to a pulsed signal. Photo emissions from the light source are coupled to a material under test. Resonant fluorescent emissions from the material are coupled to a photodiode. Current from the photodiode is coupled into an amplifier system comprising a first and second amplifier stages. The first amplifier stage is gated to a low gain when the light source is turned ON and the gain is increased when the light source goes from ON to OFF. The second amplifier stage has digitally programmable offset and gain settings in response to control signals. The output of the second amplifier stage is digitized by an analog to digital converter. A controller generates the pulse control signal and the control signals.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/967,579 filed Dec. 14, 2010, published as US 2012/0145924 A1 andentitled “Fluorescence Emissions Detector,” which is hereby incorporatedby reference herein in its entirety.

TECHNICAL FIELD

This invention relates to operating an excitation source and analogfront end of a fluorescence emissions detection circuit.

BACKGROUND INFORMATION

Fluorescence is the emission of electromagnetic radiation or lightimmediately after absorbing incident radiation or light. A resonantfluorescence phenomenon occurs when the spectra or wavelength of theemission overlaps the incident or excitation source wavelength. Thedelay time between absorption and emission is minimal. The fluorescencelifetime or 1/e value is the time that is equivalent to 36.8% of theinitial intensity value of the fluorescing signal.

Detecting fluorescence requires receiving the fluorescence emission by aphoto sensitive device, converting it to an electrical signal, andamplifying the resulting electrical signal. The spectral power density(SPD) of the excitation source is multiple orders of magnitude higherthan the SPD of the emission from the fluorescing material. Because ofthis, crosstalk in a detection system is an undesired effect ofoverlapping spectra. A saturated amplifier can substantially degrade thebandwidth and linearity of the system when detecting fluorescence inmaterials where the fluorescence has a short lifetime.

A current analog of the fluorescing emission, as measured in a detectionsystem, may be represented by an exponential equation:

f (t)=A+B ₁(e ^(−t/τ1))+B ₂(e ^(−t/τ2))  (1)

In this equation (1), “A” represents a constant background signal suchas photodiode dark current or electrical noise or offsets. Thecoefficients B₁ and B₂ represent initial emission intensities and theexponents represent the time constants of individual components in thecomposite emission signal as a function of time, t.

It is desirable to accurately detect resonant fluorescence emissionsignals with time constant components ranging from 10 to 1000microseconds. A desired output signal is a decaying exponential voltagethat is the analog of a resonant fluorescence signal with minimaldistortion due to instrumental artifacts and overlapping spectra. It isalso desirable to have a light excitation source such as a single lightemitting diode (LED) or multiple LEDs with sufficient power and asufficiently short duration or pulse width. It is also desirable to havea driver for the light source that is compatible with CMOS logic levelssuch that a single general purpose I/O pin from a microcontroller or DSPcan control the turn-ON and turn-OFF timing of the LED.

There is a desire for a resonant fluorescence detection system that issimple, reliable, low cost, and able to be configured with high volumeLED light sources and photodiode detectors. It is further desirable tohave digitally controlled gain, offset, and gating to prevent amplifiersaturation and non-linearity in addition to enabling the normalizationof detector performance to reduce device to device variance for a widevariety of applications including variances in taggant loading, whichmay result in lower or higher emission intensity.

SUMMARY

Aspects of the present invention detect resonant fluorescent emissionsfrom a test material employing a pulsed light source that is gated ONand OFF with a pulsed control signal. The light pulse is coupled intothe material under test. Resonant fluorescence emissions from the testmaterial are coupled into a photodiode that converts the radiation to anelectrical current. The current is amplified in an amplifier system,which may comprise first and second stage amplifiers. The first stageamplifier may be a current-to-voltage converter. The gain of the firstamplifier stage is reduced when the light source is gated ON andincreased when the light source is gated OFF. The source impedances onthe inputs of the first stage amplifier are balanced so the effectiveinput resistance and capacitance are substantially the same when theamplifier is switched between low gain and high gain to minimize effectsof charge injection from the switches on the amplifier response time.The output of the first amplifier is coupled to the second stageamplifier which may have programmable offset and gain digitallycontrolled in response to control signals. The output of the secondstage may be digitized by an analog-to-digital converter and analyzed.The pulse control signal and the amplifier control signals may begenerated by a controller.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a system block diagram of a resonant fluorescenceemissions detection system;

FIG. 2 illustrates a circuit diagram of a first stage transconductanceamplifier;

FIG. 3A illustrates a circuit diagram of one embodiment of a secondstage amplifier with adjustable gain and offset;

FIG. 3B illustrates a circuit diagram of another embodiment of a secondstage amplifier with programmable gain and offset;

FIG. 4 illustrates a flow diagram of method steps according toembodiments herein; and

FIG. 5 illustrates a circuit diagram of an LED driver and an electronicswitch driver useable with embodiments herein.

DETAILED DESCRIPTION

The analog front end of a single element resonant fluorescence detectionsystem comprises an excitation circuit and a single photodiode connectedto a photodiode amplifier. The analog front end (AFE) establishes amaximum performance of the system with regards to bandwidth,signal-to-noise ratio, linearity, and dynamic range. The outputcomprises a decaying exponential voltage that is the analog of aresonant fluorescence signal with minimal distortion due to instrumentalartifacts and overlapping spectra.

Fluorescence is the emission of electromagnetic radiation or lightimmediately after absorbing incident radiation or light. The resonantfluorescence phenomenon occurs when the spectra or wavelength of theemission overlaps the incident or excitation source wavelength. Thedelay time between absorption and emission is minimal. The fluorescencelifetime or 1/e value is the time that is equivalent to 36.8% of theinitial intensity value of the fluorescing signal.

The excitation circuit is designed to drive a single light emittingdiode (LED), or multiple LEDs, with sufficient power and with asufficiently short duration or pulse width. The input of the excitationcircuit may be designed to be compatible with CMOS logic levels suchthat a single general purpose I/O pin from a microcontroller or DSP maycontrol the turn-ON and turn-OFF timing of the LED. A typical excitationpulse width may be in the range of 1 millisecond.

A photodiode is biased to operate in the photoconductive mode andconnected to a two-stage amplifier. The first stage amplifier may be atransimpedance amplifier designed to convert the photodiode current to avoltage. The second stage amplifier may be a non-inverting amplifierthat further amplifies and conditions the photodiode current.

The digital input signal to the excitation circuit also drives or gatesanalog switches, which substantially reduces the gain of thetransimpedance amplifier when the excitation LED is ON or radiating.Reducing amplifier gain as a function of excitation source statusdecreases non-linearity due to amplifier saturation or cross-talk fromthe excitation source, crosstalk being an undesired effect ofoverlapping spectra. The spectral power density (SPD) of the excitationsource is multiple orders of magnitude higher than the SPD of thefluorescing material emission. A saturated amplifier may substantiallydegrade the bandwidth and linearity of the system when detectingmaterials with short lifetimes, thus impairing, complicating orincreasing cost of authentication system and method. Further details aredescribed relative to the figures that follow.

FIG. 1 illustrates a block diagram of resonant fluorescence detectionsystem 100. System 100 may be utilized to determine if a monitoredmaterial possesses a particular taggant which is configured with acomposition that fluoresces when irradiated with a particular wavelengthof light. Microcontroller 102, in response to control steps programmedwithin, provides a pulsed signal 109 with a predetermined pulse width(e.g., 1 millisecond) to light emitting diode (LED) driver 103. LEDdriver 103 is configured to turn ON LED 104 (pulse signal 121) for atime period equal to the predetermined pulse width. When LED 104 turnsON, a pulse of light 114 excites tagged substrate 101 (an example of theaforementioned material possessing a taggant). In response to the pulseof light 114, tagged substrate 101 fluoresces after absorbing theincident light 114, thereby producing resonant fluorescence emission113. Photodiode (PD) 105 is configured as a photoconductive detector.Bias voltage (V bias) 111 reverse biases PD 105. When no light isreceived by PD 105, it conducts “dark current” under the influence ofbias voltage 111. When light (e.g., 113) impinges on PD 105 it isrendered more conductive and signal current 115, proportional to theincident light 113, flows through transimpedance amplifier (AMP1) 106,which converts current 115 to output voltage Vo 112. Output voltage Vo112 is further amplified by amplifier (AMP2) 107, thus producing outputvoltage Vout 118. Analog-to-digital (A/D) converter 108 converts Vout118 to a digital signal at output 122. A/D converter 108 may also becoupled to microcontroller 102 via signal(s) 116. Microcontroller 102receives program signals 117 and provides signal 109 and control signals110. Control signals 110 provide digital signals used to affect offsetcontrol and gain control for amplifier 107. Pulse signal 109 is used togate LED driver 103 and shutter driver 120. Shutter driver 120 is usedvia voltage-controlled analog switches within amplifier 106 (see FIG.2), to switch resistors that balance the source impedances of amplifier106 when it is switched between a high and low gain.

FIG. 2 illustrates further details of amplifier 106. Amplifier 106comprises operational amplifier U1 as a main gain element. Amplifier U1is powered by positive voltage V1 201 and previously disclosed negativebias voltage V2 111. An operational amplifier, such as U1, is designedto have a very high input impedance at its input terminals (e.g., 213and 214) and a very high open loop gain. External feedback componentsare used to provide the performance desired in amplifier 106. Aspreviously described, PD 105 is reversed biased, and substantially allof the current flow (i.e., current 115) through PD 105 is forced to flowthrough the feedback network, comprising analog switch S2, resistor R9,resistor R3, capacitor C2, and resistors R5 and R4, by action ofamplifier output voltage Vo 112. Because of the high gain between theinputs 213, 214 of amplifier U1, amplifier output voltage Vo 112“servos” to a value necessary to maintain the voltage difference betweeninput 213 and input 214 at essentially zero volts. The high inputimpedance of negative input 214 assures essentially all of the current115 flows into the aforementioned feedback network.

Since PD 105 is reversed biased, it acts much as a current source,wherein the magnitude of current that flows (i.e., current 115) dependspredominately on the amount of light energy 113 impinging on PD 105, anddoes not depend significantly on resistor R2 or bias voltage 111.

Resistors R3, R4, R9, and R5 shape the gain and, along with capacitorsC2 and C3, the frequency response of amplifier 106. It can be shown thatthe transfer function, expressed as the ratio (Vo 112/current 115), isequal to (R4+R3(1+R4/R5)) when resistor R9 is selected by switch S2. Theresistance of resistor R9 is much smaller than the resistances ofresistors R3, R5, or R3. When resistor R9 is selected, essentially allof current 115 flows in resistor R9, and the aforementioned transferfunction is essentially equal to the value of resistor R9. Thus,switching in resistor R9 when LED 104 is pulsed ON reduces the gain ofamplifier 106. Capacitor C2 is in parallel with resistor R3 and reducesthe gain of amplifier 106 as frequency increases when switch S2 isnormally open. Reducing the gain of amplifier 106 during the time LED104 is pulsed ON prevents, or at least reduces, saturation orcross-talk, since the spectral power density (SPD) of LED 104 ismultiple orders of magnitude greater than the SPD of resonantfluorescence emissions 113.

Switches S1 and S2 may be electronic switches, which may cause chargeinjection at the inputs of amplifier U1 when switching amplifier 106between a high and low gain. If different source impedances arepresented at the inputs when the gain is switched, switches S1 and S2may thereby cause amplifier 106 to have an increased settling time.Capacitances C1 and C2 are thereby sized to be substantially equal.Capacitor C3 provides compensation for gain peaking, thus improvingamplifier stability. Likewise, resistances R8 and R3 are sized to besubstantially equal, and switched resistances R9 and R1 are sized to besubstantially equal. Balancing the impedances at the input of amplifierU1 improves system performance when amplifier U1 is switched betweenhigh gain and low gain when LED 104 is gated ON and OFF.

FIG. 3A illustrates details of one embodiment of amplifier 107, whichreceives output voltage Vo 112 of amplifier 106. Amplifier 107 comprisesoperational amplifier U2 configured as a non-inverting amplifier with again set by feedback resistors R17-R20 and R12. The closed loop gain ofamplifier 107 may be shown to be greater than one and equal to the sumof resistor R12 and selected resistor(s) from R17-R20 divided byresistor R12. If analog switch S4 selects more than one resistor, thenthe resistor value used in the gain equation is the parallel combinationof the selected resistors. Analog switch S4 acts to provide programmablegain in response to digital gain control signals 110 from control logiccircuitry 102 (see FIG. 1). Amplifier 106 will undoubtedly have a DCoffset component in its output Vo 112 that it is undesirable. Byapplying a DC signal to one end of resistor R11 (signal 303), theoffset, as seen in output Vo 118, may be controlled to a desired value.FIG. 3A illustrates an embodiment where the offset signal 304 isprovided by a programmable voltage divider network comprising resistorsR10 and R13-R16 and switch S3, which may be an analog switch.Transistors T11 and T12 are configured to receive offset signal 304 andprovide a voltage follower whose output (303) voltage will go frompositive to negative to facilitate a bipolar offset voltage. Othercircuitry for generating the compensating offset voltage to node 303 maybe used and still be within the scope of the present invention.Capacitors C11 and C12 reduce the high frequency gain of amplifier 107,which may enhance the overall signal-to-noise ratio of the detector inthe bandwidth of interest.

FIG. 3B illustrates details of another embodiment for an amplifier 107,which receives output voltage Vo 112 of amplifier 106. Amplifier 107comprises operational amplifier U2 configured as a non-invertingamplifier with a gain set by feedback resistors R37-R40 and R32. Theclosed loop gain of amplifier 107 may be shown to be greater than oneand equal to the sum of resistor R32 and selected resistor(s) fromR37-R40 divided by R32. If switch S5 which may be an analog switchselects more than one resistor, then the resistor value used in the gainequation is the parallel combination of the selected resistors. SwitchS5 acts to provide programmable gain in response to digital gain controlsignals 110 from control logic circuitry 102 (see FIG. 1). Amplifier 106will undoubtedly have a DC offset component in its output Vo 112 that itis undesirable. By applying a DC signal to one end of resistor R31(signal 305), the offset, as seen in output Vo 118, may be reduced to asmall value. FIG. 3B illustrates an embodiment where the offset signal305 is provided by a voltage divider network comprising resistors R30and R34 and potentiometer P1. Resistors R30 and R34 set the coarsedivision and potentiometer P1 provides for fine adjustment of an offsetvoltage plus and minus around zero volts. It is understood thatprogrammable switch S5 may be replaced by manual switches for selectingresistors R37-R40.

FIG. 4 illustrates a flow diagram of method steps 400 according toaspects of the present invention. In step 401, a pulsed light 114 ofpredetermined duration is directed to the material under test 101. Instep 402, resonant fluorescence emissions 113 from the material 101 arereceived by a photoconductive device 105. In step 403, thephotoconductive device's current 115 is modulated by the resonantfluorescence emissions 113 and coupled to an amplifier system,comprising amplifiers 106 and 107, which converts the current changes tovoltage changes. In step 404, the gain of the first amplifier stage 106is reduced and its input source impedances are balanced during turn ONof the pulsed light source LED 104. In step 405, the gain of the firstamplifier stage 106 is increased and its input source impedances arebalanced during turn OFF of the pulsed light source LED 104. In step406, the output waveform of the second amplifier stage 107 is analyzedas a measure of the intensity of the resonant fluorescence emission.

In step 407, a test is done to determine if the dynamic range of theamplifier system has been optimized. If the dynamic range has not beenoptimized in step 408, the offset and gain of the amplifier system areadjusted to set an optimized dynamic range. If it is determined in step404 that the dynamic range of the amplifier system has been optimized,then in step 409 the waveform from the amplifier system is conditionedand used in an authentication algorithm validating the material undertest.

FIG. 5 illustrates a circuit diagram of a LED driver 103 and a shutterdriver 120 useable with embodiments herewithin. Shutter driver 120receives a pulse signal 109 from microcontroller 102. Shutter signal 119may be used to turn ON switches S1 and S2 (see FIG. 2) from theirnormally open state, thereby coupling resistor R1 to ground and couplingresistor R9 between the output 112 and the input 214 of amplifier U1.Shutter signal 119 may have a rise time (essentially determined byresistor R51 and capacitor C50) different from its fall time(essentially determined by resistor R52 and capacitor C50). The risetime determines how fast amplifier 106 switches to low gain when LED 104is turned ON and the fall time determines how quickly the gain of theamplifier is switched to a high gain after LED 104 is turned OFF.Resistor R50 limits the base current to transistor T50.

LED driver 103 may be used to generate drive signal 121 that turns ONand OFF exemplary LED 104. Transistor T51 provides the drive current toLED 104 through resistor R55 when LED signal 109 is at a positive levelreducing the loading on LED signal 109. Since the source impedance oftransistor T51 and the resistance of resistor R55 are low, the turn ONtime of LED 104 is fast. When LED signal 109 goes low to turn OFF LED104, the capacitance of LED 104 would slow its turn OFF if transistorT52 was not present. When LED signal 109 goes low, transistor T52 willturn ON, providing a low impedance discharge path (between its emitterand collector) for the signal 121 until its value drops below theemitter-to-base turn ON threshold of transistor T52. Turning LED 104 OFFquickly reduces the delay time required before the fluorescence emissionsignal is ready for analysis.

Returning to FIG. 1, an application of system 100 is to provide a signalfrom objects possessing a particular taggant for purposes ofidentification. For example, tagged substrate 101 may be an item ofvalue or critical source of information such as labels, productpackaging material, a bank note or a series of bank notes that pass bysystem 100 on a conveyor system (not shown) during manufacturing,warehousing, sorting, or retail operations. Tagged items may alsoinclude coins, stamps, and consumable medical supplies, such as reagentand glucose test strips that are targeted by counterfeiting operationsor unlicensed third party manufacturing operations. As each bank note ortagged item passes by the optical components 104 and 105, system 100 isprogrammed to pulse LED 104. This may be performed by a communication117 from the conveyor system, or a proximity detector such as aphotointerruptor with an output conditioned to be provided to controllogic 103 when each bank note is within the target area of the opticaldevices 104, 105. If a particular bank note possesses the specified andknown taggant, it will fluoresce when irradiated by light 114, and PD105 will then detect that florescence 113, eventually resulting inoutput 114 indicating the presence of the taggant. In such a system, oneor more bank notes may be “scanned” and each determined to be eitherauthentic or counterfeit. System 100 provides advantages such that thisauthentication process may be performed faster (the conveyor speed maybe increased) while maintaining a desired level of accuracy. It isunderstood that any material or device with a fluorescing taggant may bemonitored with such a system 100.

One of ordinary skill in the art will appreciate other advantages of theembodiments described herein. Embodiments of the invention do notrequire optical components such as lens and optical filters to improvethe signal-to-noise ratio of the emission signal or to isolate thephotodiode from the excitation source when it is active, thus theincreasing cost and form factor of the detector.

A single digital signal or bit may be used to turn the excitation sourceON and OFF while also controlling the gain of the first stage amplifierusing a method that minimizes amplifier settling time due to balancingthe effective charge injection of the pair of analog switches.Embodiments herein do not rely on non-linear amplifier functions, andthe photodiode bias circuit does not require a voltage clamp to maintaindesired performance.

The effective dynamic range of the invention is sufficient withamplifier power supply rail voltages as low as +/−5 VDC. Operating fromlow power supply rail voltages and reduced power enables a low noisedesign that may be powered from existing power supply nodes containedwithin a host system. For example, all power to operate a deviceinclusive of the invention and an embedded system host may be derivedfrom a universal serial bus (USB) port. The USB port contains a +5 VDCpower source. This source may be converted to −5 VDC using an integratedcircuit (not shown) designed to function as a voltage inverter or chargepump.

Distributing the overall amplifier gain in two stages enablesembodiments to have a faster sensor response time due to a lowermeasurement time constant affected by the photodiode capacitance. Thephotodiode appears as a current source in parallel with a capacitor.While increasing the reverse bias reduces the capacitance of thephotodiode, it is desirable to keep the voltage supplies (thus biasvoltage) low. Using a low gain first stage allows the resistances of thefeedback to be smaller, and thus the time constant of the response timefor a given photodiode capacitance may be reduced or minimized. Thefirst stage gain (transimpedance) may be lower while the second stagegain may be higher.

The direct current (DC) offset value referenced by “A” in equation (1)may be substantially eliminated in embodiments herein by adding thelevel shifting circuit to the second stage amplifier without increasingthe response time which is determined by the impedances of the firstamplifier stage and the photodiode capacitance.

The second stage amplifier in embodiments is a programmable gainnon-inverting amplifier with a resistor ladder to the operationalamplifier negative feedback. Having a programmable gain amplifierenables an automatic gain selection algorithm to substantially extend anoverall dynamic range of the system. The second stage amplifier may alsoinclude a programmable offset compensation circuit (digitalpotentiometer). The offset circuit may be adjusted to ensure that theanalog output signal remains above ground potential. The effects of theprogrammable offset circuit may be integrated into an automatic gainalgorithm to further maximize dynamic range for a broad set of operatingconditions. The second stage may also include an active filter (e.g.,capacitor C12 in FIG. 3A) to selectively attenuate or filter targetedfrequency bands without affecting the bandwidth of the photodiodesensor. The transient response of the system to a unit step function inembodiments is reduced by balancing the source impedances at the inputsof the first stage amplifier that uses analog switches to switch betweena high and low gain.

1-20. (canceled)
 21. A method of detecting fluorescence emission from atarget comprising: irradiating the target with a light pulse, whereinthe light pulse transitions between an on state and an off state;wherein an amplifier gain and balancing source impedance is reduced inresponse to the light pulse being in the on state; and wherein theamplifier gain and balancing source impedance is increased in responseto the light pulse being in the off state; receiving a fluorescenceemission signal in response to irradiating the target with the lightpulse, and thereby generating a current signal; conditioning the currentsignal via the amplifier, and thereby generating an output signal; andapplying an authentication algorithm to the output signal, and therebyvalidating the target.
 22. The method of claim 21, wherein the lightpulse is generated by a light excitation source in response to a pulsedsignal.
 23. The method of claim 21, wherein conditioning the currentsignal comprises converting the current signal to a voltage signal. 24.The method of claim 23, further comprising amplifying the voltagesignal.
 25. The method of claim 1 wherein the fluorescence emissionsignal is a non-resonant signal.
 26. The method of claim 21, wherein thefluorescence emission signal is a resonant signal.
 27. The method ofclaim 21, wherein the fluorescence emission signal is a quasi-resonantsignal.
 28. A system for detecting fluorescence comprising: a lightexcitation source, wherein the light excitation source is configured totransition between an activated state and a deactivated state inresponse to a pulsed signal; wherein, when in the activated state, thelight excitation source is configured to emit a light; and wherein, whenin the deactivated state, the light excitation source is configured tonot emit the light; a photodiode, wherein the photodiode is configuredto receive a fluorescence emission signal in response to the light andto generate a current signal in response to the fluorescence emissionsignal; and an amplifier, wherein the amplifier coupled to thephotodiode and is configured to generate a voltage signal in response toreceiving the current signal; wherein the amplifier is configured toprovide a first gain when the light excitation source is in theactivated state; and wherein the amplifier is configured to provide asecond gain when the light excitation source is in the deactivatedstate.
 29. The system of claim 28, further comprising a microcontrollerin signal communication with the amplifier, wherein the microcontrolleris configured to perform an authentication algorithm using the voltagesignal.
 30. The system of claim 29, wherein the amplifier is in signalcommunication with the microcontroller via a second amplifier, whereinthe second amplifier is directly connected to the amplifier; and whereinthe second amplifier is configured to generate an amplified voltagesignal in response to receiving the voltage signal.
 31. The system ofclaim 30, wherein the second amplifier is configured to provide aconfigurable gain and a configurable offset.
 32. The system of claim 28,wherein the first gain is less than the second gain.
 33. The system ofclaim 28, wherein the fluorescence emission signal is a resonantfluorescence signal.
 34. The system of claim 28, wherein thefluorescence emissions signal is a non-resonant fluorescence signal. 35.The system of claim 28, wherein the fluorescence emission signal is aquasi-resonant signal.
 36. The system of claim 8, wherein the amplifiercomprises: a positive input, wherein the positive input is selectivelycoupled to a ground potential via a first resistor/capacitor network; anegative input, wherein the negative input is coupled to a supplyvoltage via the photodiode; and an output, wherein the output is coupledto the negative input via a second resistor/capacitor network.
 37. Thesystem of claim 36, wherein a first analog switch selectively couplesresistors of the first resistor/capacitor network to the positive inputof the amplifier in response to the pulsed signal.
 38. The system ofclaim 37, wherein a second analog switch selectively couples resistorsof the second resistor/capacitor network to the negative input of theamplifier in response to the pulsed signal.
 39. The system of claim 30,wherein the second amplifier comprises: a positive input, wherein thepositive input is couple an output of the amplifier; and a negativeinput, wherein the negative input is coupled an offset voltage via afirst resistor and to a voltage divider network; wherein the voltagedivider network comprises a parallel resistor network comprisingselectively coupled parallel resistors; and wherein the voltage dividernetwork comprises: a first terminal coupled to an output of the secondamplifier; a second terminal coupled to a ground potential; and a thirdterminal coupled to the negative input of the second amplifier.
 40. Thesystem of claim 39, wherein the parallel resistor network comprisesanalog switches for selectively connecting resistors of the parallelresistor network to the negative terminal of the second amplifier,wherein the analog switches are selectable in response to a controlsignal.